of -us]) is an extinct subclass of pteraspidomorph, ostracoderm, jawless vertebrate that lived primarily in marine and estuary environments.

In a few primitive forms, such as Lepidaspis, the dorsal and ventral shields are composed of a mosaic of tiny scales.

The scales of heterostracans are histologically distinct from other vertebrates, having three layers composed of dentine and aspidine, an acellular bony tissue unique to animals of this class and Thelodonti.

Cyathaspidiformes is divided into two main subgroups: the Amphiaspidida of Early Devonian Siberia (i.e., Amphiaspis, Gabreyaspis, and Edaphaspis, grouped together with their relative Ctenaspis of Canada), and the Cyathaspidida (i.e., Cyathaspis, Poraspis, and Anglaspis), grouped together with their relative Nahanbiaspis.

Pteraspididae, Gigantaspididae and Protaspididae, in turn form the suborder Pteraspidoidei, with various genera of Protopteraspididae as sister taxa.